Homogeneity range of ɛ -phase Zn4 Sb3

Journal of Alloys and Compounds 432 (2007) 116–121

Homogeneity range of ⑀-phase Zn4 Sb3 G. Nakamoto a,∗ , N. Akai a , M. Kurisu a , I.-H. Kim a,1 , S.-C. Ur b , V.L. Kuznetsov a,2,3 b

a Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan Research Center for Sustainable ECo-Devices and Materials, Department of Materials Science and Engineering, Chungju National University, Chungju, Chungbuk 380-702, South Korea

Received 31 January 2006; received in revised form 29 May 2006; accepted 31 May 2006 Available online 20 July 2006

Abstract The ⑀-phase of Zn4 Sb3 compound has been examined by the powder X-ray diffraction (XRD) and electron-probe micro analysis (EPMA) on three ingots prepared, respectively, by gradient freeze (GF), Bridgman (BR) and solid-state synthesis (SSS) with different starting compositions. It is found that the homogeneity range of the ⑀-phase is narrow within 0.20 at.% Zn irrespective of different preparation methods and starting compositions. © 2006 Elsevier B.V. All rights reserved. PACS: 72.20.Pa Keywords: Zn4 Sb3 ; Electron-probe micro analysis; Homogeneity range; Thermoelectric material

1. Introduction The ⑀-phase of Zn4 Sb3 compound is well-known as one of the best p-type thermoelectric materials for use in the moderate temperature range because of the high dimensionless figure of merit, ZT = 1.3 at 670 K [1,2]. Since the discovery of the good thermoelectric performance in the Zn4 Sb3 compound, many efforts have been devoted to optimize the thermoelectric performance of the compound by doping, substitution of the constituents by other elements and modification of preparation methods [3–13]. However, there still remain many unresolved problems of the composition of the ⑀-phase. There have been many reports concerning the crystal structure and binary phase diagram of Zn-Sb system [14–19]. Takei has discovered the 4:3 phase and constructed the phase diagram [14]. Mayer et al. have firstly determined the crystal structure of the ⑀-phase [15]. A new phase

∗

Corresponding author. Tel.: +81 761 51 1543; fax: +81 761 51 1545. 1 On leave from Research Center for Sustainable ECo-Devices and Materials, Department of Materials Science and Engineering, Chungju National University, Chungju, Chungbuk 380-702, South Korea. 2 On leave from Division of Electronic Engineering, Cardiff University, 5 The Parade, Cardiff CF24 OYE, UK. 3 Present address: Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, UK. E-mail address: [email protected] (G. Nakamoto). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.05.118

diagram has been proposed by Izard et al. [17] to show that the ⑀-phase can be formed in the limited range of the starting composition between 56.5 and 57.0 at.% Zn. Quite recently, a modified crystal structure has been proposed by Snyder et al. [19]. They have introduced new interstitial sites of Zn atom and claimed that the ideal composition of the ⑀-phase is nearly 13:10. This composition is located in the homogeneity range reported by Izard et al. In this study, in order to determine the homogeneity range of the ⑀-phase of Zn4 Sb3 compound, three ingots of Zn4 Sb3 compound have been prepared, respectively, by different methods: gradient freeze (GF), Bridgman (BR) and solid-state synthesis (SSS) methods with various starting compositions. The spatial distribution of Zn content has been examined for these three ingots by powder X-ray diffraction (XRD) and electron-probe micro analysis (EPMA). 2. Experimental Three ingots of Zn4 Sb3 compound were prepared under different conditions as listed in Table 1. The ingot by a GF method (GF ingot) was grown with the stoichiometric starting composition of 57.14 at.% Zn. The ingot by a BR method (BR ingot) was grown with starting composition of Zn3.96 Sb3.04 (56.57 at.% Zn). An SSS method was employed to fabricate the ingot (SSS ingot) with Zn-rich starting composition by 0.5 wt.%. The detailed preparation conditions are described elsewhere

G. Nakamoto et al. / Journal of Alloys and Compounds 432 (2007) 116–121 Table 1 Preparation conditions of Zn4 Sb3 compounds

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Table 2 Lattice constants of Zn4 Sb3 compounds determined by the Rietveld analysis

Sample name

Preparation method

Starting composition

References

GF BR SSS

Gradient freeze Bridgman Solid-state synthesis

57.14 at.% Zn 56.57 at.% Zn Zn4 Sb3 + 0.5 wt.% Zn

[8] [12] [10]

[8,10,12]. An XRD measurement was performed for these three ingots with Cu K␣ radiation using a Rigaku Corporation RINT-2000. The lattice constants were determined by the Rietveld analysis using a RIETAN-2000. All the ingots were sliced into some plated pieces suitable for the EPMA measurement. The surfaces of the sliced pieces were polished by an emery paper and alumina powder to obtain clean and flat plane. The polished pieces were fixed by a carbon tape on to a sample holder to make an electrical contact between them. The EPMA measurement was done by wavelength-dispersive spectrometry with an electron beam diameter of 10 ␮m using a JEOL JXA-8621 at room temperature. The acceleration voltage and probe current were set to 20 kV and 40 nA, respectively. A linear scanning mode was employed. A calibration was made before and after a set of measurements of each piece by using ZnTe and InSb compounds as standard samples.

3. Results and discussion 3.1. XRD profiles Fig. 1 shows the powder XRD profiles of the GF (a), BR (b) and SSS (c) ingots. Three different parts, indicated by shaded circles, are measured for each ingot. For the GF ingot, as shown in Fig. 1(a), the diffraction peaks at the bottom and middle parts can be indexed by assuming the rhombohedral structure proposed by Snyder et al. [19]. On the other hand, a reflection from an impurity Zn appears at the top part as indicated by an arrow. In the BR and SSS ingots, it is found that all the parts are single phase.

Sample name

Measurement part

Lattice constants

Impurity phase

˚ a (A)

˚ c (A)

GF

Top Middle Bottom

12.2293(5) 12.2282(6) 12.2339(5)

12.4254(4) 12.4242(5) 12.4287(4)

Zn – –

BR

Edge Middle Center

12.2338(6) 12.2261(5) 12.2287(6)

12.4291(5) 12.4197(4) 12.4231(5)

– – –

SSS

Edge Middle Center

12.2254(4) 12.2238(3) 12.2311(3)

12.4193(4) 12.4185(2) 12.4262(3)

– – –

It should be also noted that the relative intensity of the diffraction peaks is ingot-dependent for these three ingots, GF, BR and SSS. Furthermore, position dependence of the relative intensity is found in the GF and BR ingots, whereas in the SSS ingot the XRD profiles are similar for the three parts. It is supposed that the difference in the relative intensity may reflect the difference in the degree of Zn occupation into the 36f site, depending on preparation condition. The lattice constants of the three ingots determined by the Rietveld analysis are listed in Table 2. No obvious correlation is found between the lattice constants and measurement part for all the ingots. The lattice constants are distributed within 0.09% for both the a- and c-axes in these three ingots. 3.2. EPMA 3.2.1. Gradient freeze ingot (GF ingot) Fig. 2 shows the spatial distribution of Zn content along the crystal growth direction z at different radial positions x for the ingot prepared by a GF method with the stoichiometric starting composition of 57.14 at.% Zn.

Fig. 1. Powder X-ray diffraction profiles of Zn4 Sb3 compounds: the GF (a), BR (b) and SSS (c) ingots.

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Fig. 3. Frequency distribution of Zn content of the ⑀-phase for Zn4 Sb3 ingot prepared by a gradient freeze (GF) method with the stoichiometric starting composition. The solid curve represents the result of a least square fitting by assuming a Gaussian function.

each position x, as will be shown later. Therefore, we conclude that the composition of the ⑀-phase is homogeneous along z for different x. Next, we have examined the distribution width of Zn content at each x. A Gaussian function is assumed for the distribution of Zn content. The result is summarized in Table 3. All the positions give almost constant average Zn content and distribution width, indicating homogeneous composition of the ⑀-phase throughout the GF ingot. Thus, the average of the Zn content and distribution width are evaluated by using all the data. Fig. 3 shows the frequency distribution of Zn content for the GF ingot. The solid curve represents the result of a least square fitting by assuming a Gaussian function. The composition and distribution width (half width of half maximum) of the ⑀-phase are determined to be 58.22 and 0.14 at.% Zn, respectively.

3.2.2. Bridgman ingot (BR ingot) Spatial distribution of Zn is displayed in Fig. 4 in the plane perpendicular to the crystal growth direction for the ingot prepared by a BR method. The starting composition of the BR ingot was shifted to the Sb-rich side at a ratio of 3.96:3.04 according to the composition range reported by Izard et al. Very homogeneous distribution is found in the plane without any deviation and scattering. The frequency distribution of Zn content is plotted in Fig. 5 in the plane for the BR ingot. The solid curve represents the result of a least square fitting by assuming a Gaussian function. The composition and distribution width of the ⑀-phase is determined to be 58.13 and 0.16 at.% Zn, respectively. Fig. 2. Spatial distribution of Zn content along the crystal growth direction z at various radial position x for Zn4 Sb3 ingot prepared by a gradient freeze (GF) method with the stoichiometric starting composition.

First, we have performed a least square fitting for the Zn content by assuming a linear variation with z. The obtained fitting parameters are given in Fig. 2. Although all the positions of the ingot at different x have a negative slope of Zn content along z ranging from −0.0005 to −0.0103 at.% Zn/mm, these values are negligibly smaller than the distribution width of Zn content at

Table 3 Average Zn content and distribution width of the ⑀-phase at different radial position x for Zn4 Sb3 ingot prepared by a GF method x (mm)

Zn content (at.%)

Distribution width (at.%)

0.0 1.0 2.0 3.0 4.0

58.21 58.23 58.26 58.19 58.18

0.16 0.09 0.10 0.16 0.19

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3.2.3. Solid-state synthesis ingot (SSS ingot) Fig. 6 shows spatial distribution of Zn in the plane perpendicular to the pressing direction for the SSS ingot. Larger scattering to the Zn-rich side is found. This ingot was prepared by an SSS method with relatively high Zn starting composition to compensate the loss of Zn by evaporation during hot-pressing process. Thus, it is suggested that even after hot-pressing process the excess Zn still remains as an impurity phase with a fine size comparable to or smaller than the electron beam diameter of 10 ␮m employed in the EPMA measurement. The frequency distribution is displayed in Fig. 7. The composition and distribution width of the ⑀-phase are obtained to be 58.33 and 0.24 at.% Zn, respectively. 3.3. Homogeneity range The composition and distribution width of the ⑀-phase Zn4 Sb3 compound are obtained to be 58.22 and 0.14 at.% Zn for the GF ingot, 58.13 and 0.16 at.% Zn for the BR ingot and 58.33 and 0.24 at.% Zn for the SSS ingot, respectively. We note that the composition is richer in Zn for the ingot with richer starting Zn composition. However, the difference in the composition among the three ingots is so small within the distribution width that each ingot has. Furthermore, for the SSS ingot, the existence of an impurity Zn with a fine size may be supposed to distribute in the ingot as can be seen in Fig. 6. Therefore, we can conclude that the homogeneity range of the ⑀-phase is not more than 0.20 at.% Zn for these three ingots irrespective of the wide variety of the starting compositions and different preparation methods. We should also notice that the determined Zn concentration of 58.13–58.33 at.% Zn is larger than the starting one as well as that of 56.5–57.0 at.% Zn reported by Izard et al. [17]. Two possible explanations for the discrepancy of the composition of the ⑀-phase are as follows:

Fig. 4. Spatial distribution of Zn content in the plane perpendicular to the crystal growth direction for Zn4 Sb3 ingot prepared by a Bridgman (BR) method.

Fig. 5. Frequency distribution of Zn content of the ⑀-phase for Zn4 Sb3 ingot prepared by a Bridgman (BR) method. The solid curve represents the result of a least square fitting by assuming a Gaussian function.

(1) The homogeneity range of 56.5–57.0 at.% Zn for the ⑀-phase reported by Izard et al. [17] was given in terms of starting composition. On the other hand, the composition of the ⑀phase in this study is determined directly for grown ingots. If the homogeneity range of the 4:3 phase is assumed to be dependent on temperature, then the final composition of the 4:3 phase may be different from the starting one. (2) The accuracy of the composition determined by an EPMA possibly depends on the measurement conditions such as choice of standard material, accelerating voltage of an electron beam, probe current and so on. However, the 4:3 single phase is obtained and no trace of an impurity Zn is observed in both the XRD and EPMA profiles for the BR ingot prepared with poorer Zn starting composition than the stoichiometric one. On the other hand, the impurity Zn is included in the GF and SSS ingots prepared with the stoichiometric and richer Zn compositions, respectively. These facts suggest that the 4:3 single phase can be obtained at poorer Zn composition and, therefore, the determined composition ranging from 58.13 to 58.33 at.% Zn of the 4:3 phase may be overestimated in this study.

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Fig. 7. Frequency distribution of Zn content of the ⑀-phase for Zn4 Sb3 ingot prepared by a solid-state synthesis (SSS) method. The solid curve represents the result of a least square fitting by assuming a Gaussian function.

4. Conclusion Powder X-ray diffraction and electron-probe micro analysis have been performed on three Zn4 Sb3 ingots prepared by different methods to examine the composition of the ⑀-phase Zn4 Sb3 compound. In the ingots prepared by gradient freeze and Bridgman methods, an uniform distribution of Zn content is observed throughout the ingots. On the other hand, a large scattering of Zn content is found in the solid-state synthesis ingot. It is revealed that the composition of the ⑀-phase is limited between 58.13 and 58.33 at.% Zn irrespective of different preparation methods and starting compositions. This composition range is comparable to the distribution width. This fact strongly indicates that the homogeneity range of the ⑀-phase is narrow within 0.20 at.% Zn and independent of preparation condition. References

Fig. 6. Spatial distribution of Zn content in the plane perpendicular to the pressing direction for Zn4 Sb3 ingot prepared by a solid-state synthesis (SSS) method with starting composition of Zn4 Sb3 + 0.5 wt.% Zn.

Therefore, the comparison of the composition is valid only among the three ingots evaluated under the same condition in this study and it is difficult to discuss the difference in composition between Izard’s and our results. Further systematic and careful study is required to determine the real composition of the ⑀-phase.

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